1. Field of the Invention
The present invention relates to a method for manufacturing a semiconductor device and, more particularly, to a method for manufacturing a T-gate useful for reducing a gate resistance, improving through-put, and simplifying an MMIC (monolithic microwave integrate circuit) process.
2. Discussion of the Related Art
One of methods for improving frequency characteristics of a MESFET device has been proposed in which a gate length is shortened. However, if the gate length is shortened, a gate serial resistance is increased, thereby degrading the frequency characteristics of the device. Hence, a method for manufacturing a T-gate to overcome the above-stated disadvantage has been proposed in which a contact area of a gate and a semiconductor substrate is minimized and a cross-sectional area of the gate is increased.
conventional methods for manufacturing a T-gate will be explained with reference to the accompanying drawings.
FIGS. 1a to 1e are cross-sectional views showing process steps of a method for manufacturing a tri-layer T-gate by using ultraviolet ray exposure. FIGS. 2a to 2e are cross-sectional views showing process steps of a conventional method for manufacturing a bi-layer T-gate by using electron beam.
Referring to FIG. 1a, in the method for manufacturing a T-gate by using ultraviolet ray, a first PMMA layer 2, a PMIPK layer 3, and a second PMMA layer 2a, which are a kind of polymer, are successively deposited on a semiconductor substrate 1 by using a spin-plating method. At this time, each of them is formed to have a different thickness due to its sensitivity of ultraviolet ray. The thicknesses of the first PMMA 2, the PMIPK 3, and the second PMMA 2a are respectively 0.1 .mu.m, 0.9 .mu.m, and 0.3 .mu.m.
When as much as 1.3 .sup.MJ /cm.sup.2 of ultraviolet (UV) is exposed as shown in FIG. 1b, the first PMMA 2, the PMIPK 3, and the second PMMA 2a are removed by different amounts of predetermined portions respectively, as shown in FIG. 1c. That is to say, sensitivities of the three layers are different from one another according to the thicknesses so that, when UV exposure and development is performed by using a mask based on the second PMMA layer 2a, the first PMMA layer 2 among the three layers receives the smallest amount of UV exposure energy and even reflects the UV. Thus, the PMIPK layer 3 is removed most by the reflected UV from the first PMMA layer 2, whereas the first PMMA layer 2 is removed most slightly due to its lowest sensitivity of UV.
Referring to FIG. 1d, if a 6000 Angstrom thick metal layer is deposited on the entire surface inclusive of the surface of the substrate 1 exposed by UV exposure, a gate electrode 4a is patterned. Then, all layers except the gate electrode 4a are removed with a lift-off method, thereby completing the conventional T-gate by using UV.
A method for manufacturing a bi-layer T-gate by using electron beam will be explained with reference to the accompanying drawings.
Referring to FIG. 2a, a PMMA 12 and a PMAA 12a are successively formed on a semiconductor substrate 11 to have different thicknesses. At this time, the thickness of the PMMA 12 and the PMAA 12a are 0.2 .mu.m and 0.8 .mu.m respectively.
Subsequently, electron beam is exposed by utilizing E-beam lithography equipments, as shown in FIG. 2b , thereby differently patterning the PMMA layer 12 and the PMAA layer 12a, as shown in FIG. 2c.
Referring to FIG. 2d, a metal layer 13 is formed on the entire surface inclusive of the exposed surface of the substrate 11, so as to form a gate electrode 13a. Thereafter, the PMMA layer 12 and the PMAA layer 12a and the metal layer 13 except the gate electrode 13a are all removed by utilizing a lift-off method so that the gate electrode 13a are left alone, thereby completing a conventional T-gate by using electron beam.
Referring to FIGS. 3a to 3c, there is illustrated a method for manufacturing a T-gate by using electron beam.
First, a PMMA layer 21 and a PMAA layer 22 are patterned by a small dose of exposed electron beam and by a significant dose of exposed electron beam respectively, as shown in FIG. 3a.
Referring to FIG. 3b, there is illustrated a pattern form of the PMMA layer 22 after twice of electron beam exposures.
Referring to FIG. 3c, there is illustrated a pattern form of the PMMA layer 22 after three times of electron beam exposures.
Conventional methods for manufacturing a T-gate have the following problems. A length of a gate electrode is determined by an expose energy of UV or electron beam. Further, it is difficult to make a precise pattern (critical dimension (CD)) of a first PMMA layer because there are two or three layers over the first PMMA layer. Furthermore, when electron beam is utilized, through-put becomes inferior. Finally, if any physical damage is imposed on the gate electrode while carrying out a lift-off, the gate electrode may be impaired